<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>

<div id="app">
   姓: <input  type="text" v-model="xing"/><br/>
   名: <input  type="text" v-model="ming"/><br/>
   全名:<input  type="text" v-model="fullName"/>
</div>
<script src="js/vue.js"></script>
<script>
    const app = new Vue({
        el: '#app',
        data: {
          xing:'',
          ming:''
        },
        computed: {
            fullName(){
                return this.xing + ' ' + this.ming
            }
        }
    })
</script>
</body>

</html>